Casing for a wellbore that has just been drilled is assembled at the surface as joints are added and the string is lowered into the wellbore. As the joints are added at the surface on the rig floor, it is desirable to fill the casing. Filling the casing before it is run into the wellbore prevents pressure imbalances on the casing as it is being advanced into the wellbore. Additionally, once the casing is filled, it may be desirable to circulate through the casing as it is being run into the wellbore. Prior devices have been developed to fill the casing and to circulate it. These devices used in the past are illustrated in U.S. Pat. Nos. 4,997,042; 5,191,939; and 5,735,348. These devices illustrated in these patents employed sealing elements which would seat against the inside of the casing, followed by a mechanical setdown force which opened ports to allow for circulation. Seals between a mandrel and a movable sleeve were also needed to retain a sealed connection to allow circulation. Filling in these devices was accomplished by displacement of a valve member past a lateral port to expose the lateral port to allow the casing to fill. One of the problems with the prior designs is that excessive erosion occurred at the valve member used for filling the casing, undermining its reliability. Additionally, these previous designs require at least two separate valves, one for filling the casing and the other for circulating the casing. In order to circulate with the prior designs, not only did a sealing element have to get a good sealing grip on the inside of the casing, but also the circulating ports had to be mechanically exposed using setdown weight. The configuration and nature of the operation of these prior designs made them prone to erosion. Additionally, these previous designs require additional valve components to allow pressure equalization when the pumps are stopped after circulation.
Accordingly, it is an object of the present invention to provide a system that simplifies the construction of the apparatus useful for filling aid circulating casing. Accordingly, the fill and circulation valves have been designed as a singular unit which substantially provides a large flowpath to minimize erosive effects and simplify the operation. Another object of the apparatus is to eliminate the use of any additional valves required for pressure equalization when the pumps are turned off and to simplify the design and the cost of constructing the apparatus. To facilitate the operation of the apparatus, the single valve has been configured to easily open fully. These and other objectives accomplished by the apparatus will become more apparent from a review of the detailed description below.